Railcars are constructed in such a manner as to be provided with a pair of wheel axles. One wheel axle is positioned at the front and the other wheel axle at the back of the railcar. Normally, the wheel axles are set up so that they can rotate freely within a frame of a railcar vehicle. A speed sensor, which is intended to detect the speed of rotation of the wheel axle as a means of controlling the skidding of the wheels, is installed between the wheel axle and the frame. This is achieved by utilizing a mechanism for attaching the speed sensor to the vehicle frame in such a manner that a predetermined gap is maintained between the sensor and one or more teeth of a rotary body, the rotary body being attached to an end of a wheel axle.
An example of a known speed sensor attachment mechanism of the above mentioned type suitable for the construction of a passenger railroad car is shown in FIG. 3. In this construction, a wheel axle 51, which supports wheels not shown in the drawings, consists of a support axle portion 51a, which is intended to support a wheel; a threaded portion 51b, onto which a bearing cap 53 has been screwed; and an axle end-portion 51c of [the] wheel axle 51. All of these parts are identical on both ends of wheel axle 51. A bearing 52 is inserted into a journal box 70 which is provided on a side of the frame of the railroad car, and is fixed in place by bearing cap 53. A rotary body 54 which rotates together with the wheel is fitted onto end portion 51c of wheel axle 51. Rotary body 54 has an outer peripheral surface on which a gear wheel having teeth and valleys 54a is formed. Gear wheel 54a is installed with a predetermined gap .delta. with respect to speed sensor 55. Speed sensor 55 is rigidly attached to journal box 70 through an appropriate bracket 70a.
Gear teeth and valleys 54a are capable of exerting an effect on the force lines of an electromagnetic field. Rotary body 54 has a through opening 54b which is fitted over end portion 51c of wheel axle 51 and is connected to bearing cap 53 by a bolt 56, whereby rotary body 54 can rotate together with wheel axle 51. Speed sensor 55 may comprise a conventional electromagnetic sensor. Such a sensor is rigidly attached on both sides of journal box 70 of the railcar frame. The end of the aforementioned electromagnetic sensor is installed so that it can determine the number of revolutions of wheel axle 51 by detecting variations in the magnetic field caused by the interaction of teeth and valleys 54a with the end of the sensor. The above construction requires that a gap .delta. of a predetermined magnitude be maintained between teeth and valleys 54a and speed sensor 55. If gap .delta. were to change, it would be impossible to perform measurements because of fluctuations in the output of speed sensor 55. In the event that the rotary body 54 is attached to end portion 51c of wheel axle 51, it is required that its mounting be carried out with great accuracy. Therefore, in newly constructed railway cars of the latest models, it is required that the above mentioned ends of the wheel axles be machined with extremely high accuracy to satisfy the given requirements.
However, when the speed sensor mechanism is attached to the axles of conventional railcars of the type already in use, ends 51c of wheel axle 51, even though they are coaxial, as a result of variations in the diameter D1 of the axle, fail to satisfy the above mentioned accuracy requirements. These axle ends therefore have to be machined in such a way that the deviations from the above specifications will be within the range of .+-.0.3 mm. For this reason, even if ends 51c have a tolerance identical to that of through opening 54b of rotary body 54, in some cases a certain amount of play may occur in the overlapping portion between the two, whereby the aforementioned gap .delta. between the end[s] of speed sensor 55 and the ends of gear wheel 54a may vary, making accurate measurement of wheel rotation impossible.
It has been proposed to solve this problem by measuring the actual finished dimensions at the end 51c of wheel axle 51, and then subject through opening 54b of rotary body 54 to such treatment as will result in dimensions that will match the actually measured dimensions. An alternative proposed solution involves retreating or refinishing the end 51c of wheel axle 51. The first method calls for an increase in costs, whereas the second method calls for temporarily dismantling the wheel axle from the railway car.
Because those railway cars which have the wheel construction of the type shown in FIG. 4 do not by themselves satisfy the requirements for passenger comfort, the construction of wheel axles for such cars is normally provided with a saddle 59 installed on a bearing 58 which is installed on a periphery of wheel axle 57, which in turn is inserted into wheel 61. Car frame 60 is supported by saddle 59. The gap which exists between bearing 58 and saddle 59 is not constant, and a large amount of play occurs between saddle 59 and car frame 60. In view of the above construction of the wheel axles, the speed sensor needs to be attached to frame 60 which constitutes a stable stationary element. However, since saddle 59 is present between car frame 60 and bearing 58, a gap on the order of some millimeters occurs between frame 60 and rotary body 54 attached to end 51c of wheel axle 57. Therefore, such a construction makes it practically impossible to install rotary body 54 in such a way as to eliminate fluctuations in the magnitude of gap .delta. between gear wheel 54a of rotary body 54 and the end of the speed sensor 55 attached to the side of vehicle frame 60. In FIG. 4, reference numeral 62 designates a bolster anchor, reference numeral 63 indicates a journal box block, and reference numeral 64 represents a bolster.